i got some M4A audio files that i would convert in MP3's. Now my question is:

Is a M4A file with 256kbps the equivalent to a MP3 File with 320kbps. if it is so, can i convert the M4A File to a MP3 with a constant bitrate of 320 kbps? or should i convert it to the same bitrate of 256kbps??

Then you can make a copy and play around with conversion settings until you find something that sounds indistinguishable from the original, or acceptably close.I'd store the transcoded version as e.g. filename.aac.mp3 to flag for all eternity that this was the transcoded one -- then if at a point you have devices which play all formats, you know which files to delete without extensive testing.